As a cylinder head cooling structure, a type is known in which a longitudinal passage is provided on an intake side and an exhaust side of the cylinder head and extends in a longitudinal direction (hereinafter, also referred to as a vertical direction) of the engine. As such type of cooling structure, a structure is also known in which a plurality of horizontal passages are provided between cylinders of the cylinder head and extend in a transverse direction (hereinafter, also referred to as a horizontal direction) of the engine (for example, see Patent Literature 1).